The present invention relates to a high-performance ferrite magnet having substantially a magnetoplumbite crystal structure extremely useful for wide ranges of magnet applications such as rotors for automobiles or electric apparatuses, magnet rolls for photocopiers, etc., particularly to a high-performance ferrite magnet having a microstructure having a higher coercivity iHc (or higher coercivity iHc and residual magnetic flux density Br) than those of the conventional ferrite magnets and optionally a high squareness ratio Hk/iHc, and a method for producing such a high-performance ferrite magnet.
Ferrite magnets are widely used in various applications including rotors of motors, electric generators, etc. Recently, ferrite magnets having higher magnetic properties are required particularly for the purposes of miniaturization and reduction in weight in the field of rotors for automobiles and increase in performance in the field of rotors for electric apparatuses.
High-performance sintered magnets such as Sr ferrite or Ba ferrite are conventionally produced through the following processes. First, iron oxide is mixed with a carbonate, etc. of Sr or Ba and then calcined to cause a ferritization reaction (ferrite-forming reaction). The resultant calcined clinker is coarsely pulverized, mixed with SiO2, SrCO3, CaCO3, etc. for controlling sintering behavior and Al2O3, Cr2O3, etc. for controlling iHc, and then finely pulverized to an average diameter of 0.7-1.2 xcexcm in a solvent. A slurry containing the finely pulverized ferrite-forming material is wet-molded while being oriented in a magnetic field. The resultant green body is dried, sintered and then machined to a desired shape. To increase the properties of the ferrite magnets produced by such a method, there are the following five methods available.
The first method is a fine pulverization method. When the size of crystal grains in the sintered body is close to about 0.9 xcexcm, a critical single magnetic domain diameter of a magnetoplumbite (M)-type Sr ferrite magnet, its iHc is maximum. Accordingly, fine pulverization may be carried out to an average diameter of 0.7 xcexcm or less, for instance, taking into consideration the crystal grain growth at the time of sintering. This method is, however, disadvantageous in that finer pulverization leads to poorer water removal at the time of wet molding, resulting in poorer production efficiency.
The second method is to make the sizes of the crystal grains in the sintered body as uniform as possible. Ideally, the sizes of the crystal grains are made as uniformly as possible equal to the above critical single magnetic domain diameter (about 0.9 xcexcm), because crystal grains larger than or smaller than this size have low iHc. Specific means for achieving high performance in this method is to improve a particle size distribution of fine powder. In commercial production, however, other pulverization apparatuses than ball mills, attritors, etc. cannot be used, naturally posing limitations in the level of improvement in magnetic properties by fine pulverization. Also, an attempt was recently published to produce fine ferrite powder having a uniform particle size by a chemical precipitation method. Such method is, however, not suitable for industrial mass production.
The third method is to improve crystal orientation affecting magnetic anisotropy. Specific means in this method is to improve the dispersion of ferrite particles in a fine powder slurry by adding a surfactant, or to increase the intensity of a magnetic field at the time of orientation, etc.
The fourth method is to improve the density of a sintered body. A Sr ferrite sintered body has a theoretical density of 5.15 g/cc. Sr ferrite magnets commercially available at present have densities ranging from 4.9 g/cc to 5.0 g/cc, corresponding to 95-97% of the theoretical density. Though improvement in Br is expected by increasing the density of a ferrite magnet, a higher density than the above level needs such density-increasing means as HIP, etc. However, the use of such density-increasing means leads to increase in the production cost of ferrite magnets, depriving the ferrite magnets of advantages as inexpensive magnets.
The fifth method is to improve a saturation magnetization "sgr"s or a crystal magnetic anisotropy constant of a ferrite compound per se, which is a main component (main phase) of the ferrite magnet. It is likely that the improvement in the saturation magnetization as directly leads to improvement in the residual magnetic flux density Br of the ferrite magnet. It is also likely that the improvement in the crystal magnetic anisotropy constant leads to improvement in the coercivity iHc of the ferrite magnet. Though research is being carried out on W-type ferrite having a higher saturation magnetization than that of the conventional ferrite compound having an M-type crystal structure, the W-type ferrite has not been subjected to mass production because of difficulty in the control of a sintering atmosphere.
Widely used at present among the above methods for improving the properties of ferrite magnets are the first to fourth methods, though it is difficult to drastically improve the properties of ferrite having a main phase expressed by SrO.nFe2O3 by the first to fourth methods for the reasons described below. The first reason is that the above first to fourth methods include conditions lowering productivity or steps difficult to carry out from the aspect of mass production. The second reason is that further improvement in magnetic properties, particularly Br, is extremely difficult because they are close to the theoretically highest level.
Next, as a result of investigation of a hexagonal magnetoplumbite sintered ferrite magnet described in Japanese Patent Laid-Open No. 9-115715, it has been found that higher iHc cannot easily be achieved.
It may be considered as a specific means for the above fifth method to mix ferrite expressed by AO.nFe2O3, wherein A is Sr and/or Ba, with other types of metal compounds such as metal oxides to replace part of A and Fe elements in the ferrite with other elements thereby improving the magnetic properties of the ferrite.
The magnetism of the magnetoplumbite ferrite magnet is derived from a magnetic moment of Fe ions, with a magnetic structure of a ferri-magnet in which magnetic moment is arranged partially in antiparallel by Fe ion sites. There are two methods to improve the saturation magnetization in this magnetic structure. The first method is to replace the Fe ions at sites corresponding to the antiparallel-oriented magnetic moment with another element, which has a smaller magnetic moment than Fe ions or is non-magnetic. The second method is to replace the Fe ions at sites corresponding to the parallel-oriented magnetic moment with another element having a larger magnetic moment than Fe ions.
Also, increase in a crystal magnetic anisotropy constant in the above magnetic structure can be achieved by replacing Fe ions with another element having a stronger interaction with the crystal lattice. Specifically, Fe ions are replaced with an element in which a magnetic moment derived from an orbital angular momentum remains or is large.
With the above findings in mind, research has been conducted for the purpose of replacing Fe ions with various elements by adding various metal compounds such as metal oxides. As a result, it has been found that Mn, Co and Ni are elements remarkably improving magnetic properties. However, the mere addition of the above elements would not provide ferrite magnets with fully improved magnetic properties, because the replacement of Fe ions with other elements would destroy the balance of ion valance, resulting in the generation of undesirable phases. To avoid this phenomenon, ion sites of Sr and/or Ba should be replaced with other elements for the purpose of charge compensation. For this purpose, the addition of La, Nd, Pr, Ce, etc. is effective, resulting in magnetoplumbite ferrite magnets having high Br or high Br and coercivity.
When compounds of rare earth elements such as La and compounds of M elements such as Co are added to produce high-performance ferrite magnets by the fifth method, it is usual to carry out the addition of such compounds before the calcination, namely before the ferritization reaction. Such addition method is called herein xe2x80x9cprior-addition method.xe2x80x9d Though the ferrite magnets formed by the prior-addition method have high Br and high iHc, the squareness ratio Hk/iHc tends to remarkably decrease as the amounts of these elements added increase, particularly when R is La and M is Co. The tendency of decrease in the squareness ratio Hk/iHc by the prior-addition method is also appreciated in the case of R=La, M=Co+Zn, or M=Co+Mn. Because the critical demagnetizing field intensity decreases by decrease in the squareness ratio Hk/iHc, the ferrite magnets are likely to lose its magnetization. How easily the ferrite magnets lose their magnetization is critical particularly when the ferrite magnets are assembled in magnetic circuits for rotors, etc. Ferrite magnets with higher squareness ratio Hk/iHc are thus desired.
Therefore, high-performance ferrite magnets satisfactory both in a coercivity iHc (or coercivity iHc and residual magnetic flux density Br) and in a squareness ratio Hk/iHc are desired.
Accordingly, an object of the present invention is to provide a high-performance ferrite magnet having substantially a magnetoplumbite crystal structure, which has higher coercivity iHc (or higher coercivity iHc and residual magnetic flux density Br) than those of conventional ferrite magnets and also has high squareness ratio Hk/iHc, thus useful in wide varieties of magnet applications such as rotors for automobiles and electric appliances, magnet rolls for photocopiers, etc., and a method for producing such a high-performance ferrite magnet.
Another object of the present invention is to provide a high-performance ferrite magnet having substantially a magnetoplumbite crystal structure, which has higher coercivity iHc (or higher coercivity iHc and residual magnetic flux density Br) and higher squareness ratio Hk/iHc than those of conventional ferrite magnets, and also has a microstructure in which the concentration of an R element is high in crystal grain boundaries, and a method for producing such a high-performnance ferrite magnet.
As a result of intense research in view of the above objects, the inventors have found that the addition of an R element and an M element by a post-addition method or a prior/post-addition method to ferrite having a basic composition represented by (A1xe2x88x92xRx)O.n[(Fe1xe2x88x92yMy)2O3], wherein A is Sr and/or Ba, R is at least one of rare earth elements including Y, and M is at least one element selected from the group consisting of Co, Mn, Ni and Zn can turn the ferrite to a higher-performance ferrite substantially having a magnetoplumbite crystal structure with a higher squareness ratio Hk/iHc and a reduced variation of shrink ratio, while keeping good coercivity iHc (or good coercivity iHc and residual magnetic flux density Br). The present invention has been completed based upon this finding.
Thus, the ferrite magnet according to the first embodiment of the present invention has a basic composition represented by the following general forrnula:
(A1xe2x88x92x)O.n[(Fe1xe2x88x92yMy)2O3] by atomic ratio,
wherein A is Sr and/or Ba, R is at least one of rare earth elements including Y, M is at least one element selected from the group consisting of Co, Mn, Ni and Zn, and x, y and n are numbers meeting the following conditions:
0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)],
and
5xe2x89xa6nxe2x89xa66,
and has a substantially magnetoplumbite crystal structure, the R element and/or the M element being added in the form of a compound at a pulverization step after calcination.
The ferrite magnet according to the second embodiment of the present invention has a basic composition represented by the following general formula:
(A1xe2x88x92xRx)O.n[(Fe1xe2x88x92yMy)2O3] by atomic ratio,
wherein A is Sr and/or Ba, R is at least one of rare earth elements including Y, M is at least one element selected from the group consisting of Co, Mn, Ni and Zn, and x, y and n are numbers meeting the following conditions:
0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)],
and
5xe2x89xa6nxe2x89xa66,
and has a substantially magnetoplumbite crystal structure, the R element and/or the M element being added in the form of a compound both at a mixing step before calcination and at a pulverization step after calcination.
The ferrite magnet according to the second embodiment of the present invention has a basic composition represented by the following general formula:
(A1xe2x88x92xRx)O.n[(Fe1xe2x88x92yMy)2O3] by atomic ratio,
wherein A is Sr and/or Ba, R is at least one of rare earth elements including Y, M is at least one element selected from the group consisting of Co, Mn, Ni and Zn, and x, y and n are numbers meeting the following conditions:
2 0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)],
and
5xe2x89xa6nxe2x89xa66,
and has a substantially magnetoplumbite crystal structure, the R element and/or the M element being added in the form of a compound both at a mixing step before calcination and at a pulverization step after calcination.
In both cases, the concentration of the R element is preferably higher in boundaries than in the magnetoplumbite crystal grains. When the R element is La and the M element is Co, the ferrite magnet has a residual magnetic flux density Br of 4,100 G or more, a coercivity iHc of 4,000 Oe or more and a squareness ratio Hk/iHc of 92.3% or more at 20xc2x0 C. Also, when the R element is La and the M element is Co plus Mn and/or Zn, the ferrite magnet has a residual magnetic flux density Br of 4,200 G or more, a coercivity iHc of 3,000 Oe or more and a squareness ratio KH/iHc of 93.5% or more at 20xc2x0 C.
The method for producing a ferrite magnet according to the first embodiment of the present invention, the ferrite magnet having a basic composition represented by the following general formula:
(A1xe2x88x92xRx)O.n[(Fe1xe2x88x92yMy)2O3] by atomic ratio,
wherein A is Sr and/or Ba, R is at least one of rare earth elements including Y, M is at least one element selected from the group consisting of Co, Mn, Ni and Zn, and x, y and n are numbers meeting the following conditions:
0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)],
and
5xe2x89xa6nxe2x89xa66,
and substantially having a magnetoplumbite crystal structure,
the method comprising the steps of uniformly mixing a compound of Sr and/or Ba with an iron compound; calcining the resultant mixture; adding the R element and/or the M element in the form of a compound to the resultant calcined powder at a pulverization step thereof; and sintering the resultant mixture.
The method for producing a ferrite magnet according to the second embodiment of the present invention, the ferrite magnet having a basic composition represented by the following general formula:
(A1xe2x88x92xRx)O.n[(Fe1xe2x88x92yMy)2O3] by atomic ratio,
wherein A is Sr and/or Ba, R is at least one of rare earth elements including Y, M is at least one element selected from the group consisting of Co, Mn, Ni and Zn, and x, y and n are numbers meeting the following conditions:
0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)],
and
5xe2x89xa6nxe2x89xa66,
and substantially having a magnetoplumbite crystal structure,
the method comprising the steps of adding a compound of the R element and/or the M element at a percentage of more than 0 atomic % and 80 atomic % or less on an element basis at a step of uniformly mixing a compound of Sr and/or Ba with an iron compound; calcining the resultant uniform mixture; adding the remaining amount of the compound of the R element and/or the M element to the resultant calcined powder at a pulverization step thereof; and sintering the resultant mixture.
In both cases, added as the R element compound is preferably an oxide, a hydroxide, a carbonate or an organic acid salt of at least one element selected from the group consisting of La, Nd, Pr and Ce. Also, preferably added as the M element compound is an oxide, a hydroxide, a carbonate or an organic acid salt of at least one element selected from the group consisting of Co, Mn, Ni and Zn. It is also preferable to add only a Co compound as the M element compound.
In a case where a high-performance ferrite magnet is produced by the post-addition or prior/post-addition of the R element and the M element, the resultant ferrite magnet shows extremely suppressed tendency of decrease in a squareness ratio Hk/iHc as the amounts of the R element and the M element added (values of x and y) increase, as compared with the ferrite magnets obtained by the prior-addition method.
The post-addition or prior/post-addition of the R element and the M element may lead to the deterioration of Br and iHc and to variation in a shrinkage ratio of a sintered body. To prevent the deterioration of Br and iHc and the variation of a sintering shrinkage ratio, it is preferable to add an Fe compound at the time of pulverization after calcination, in such an amount as not to hinder the magnetic orientation of a green body along a magnetic field during a process of forming the green body. Specifically, the amount of the Fe compound post-added is preferably 0.1-11 weight % on an iron element basis, based on the total amount of Fe contained in the ferrite magnet.
It has been found that when the post-addition method is adopted, the amounts of the R element and the M element (values of x and y) increase, resulting in decrease in a molar ratio n, which in turn leads to the deterioration of Br and iHc. It has also been found that when the molar ratio n decreases, variations in the size of the resultant sintered body may take place. The mechanism of decrease in the molar ratio n is as follows: Investigation is conducted on the case of the production of a ferrite magnet by the post-addition method, in which calcined ferrite powder having a composition of SrO.5.9Fe2O3, namely SrFe11.8O18.7(molar ratio n is 5.9), is used, and a La oxide is added in the course of fine pulverization to substitute about 20% of the Sr ion site with La. In this case, in order that the ferrite magnet contains substantially the same number of Co atoms as that of La atoms under charge compensation conditions, the corresponding amount of a Co oxide is simultaneously added. Assuming that all of Co added is contained as the M phase, the resultant ferrite sintered body has the following composition:
Sr0.8La0.2Fe9.60CO0.20O15.7, namely
(Sr0.8La0.2)O.4.9[(Fe0.98Co0.02)2O3].
Thus, the molar ratio n decreases from 5.9 at the stage of a calcined powder to 4.9 by the post-addition of the La oxide and the Co oxide. When the molar ratio n becomes less than 5, the relative percentage of components corresponding to the Fe ion site bearing magnetism decreases, resulting in drastic decrease in magnetic properties. At the same time, the sintering shrinkage ratio indicating how much the size changes in the course from the green body to the sintered body drastically changes, resulting in large unevenness in the sizes of the resultant ferrite magnet products.
The molar ratio n of the calcined powder may be set large in advance, taking into consideration a decrease in the molar ratio n by the post-addition method. However, this means is not effective. Investigation will be made assuming that calcined powder having a composition of SrO.n1Fe2O3 is used, and that both a La oxide and a Co oxide are added at the time of fine pulverization to produce a high-performance ferrite magnet having the following basic composition:
(Sr1xe2x88x92xLax)O.n2[(Fe1xe2x88x92yCOy)2O3] by atomic ratio,
wherein 0.01xe2x89xa6xxe2x89xa60.4, and x/(2.6n2)xe2x89xa6yxe2x89xa6x/(1.6n2). If the values of x and y were determined such that n1=6.5, and n2=5.9, the molar ratio n2 of the resultant ferrite magnet would be 5.9, within a range (5-6) of the molar ratio n suitable for the ferrite magnet. However, the ferrite magnet shows extremely poor magnetic properties in this case. The reason therefor is that when the molar ratio n1 of the calcined powder is more than 6, undesirable phases such as xcex1-Fe2O3 other than the M phase appear in the calcined powder. Because the undesirable phases are non-magnetic phases, they reduce the magnetic orientation of a green body obtained by a wet-molding method in a magnetic field. Thus, when the molar ratio n, of the calcined powder exceeds 6, Br, a squareness ratio Hk/iHc, etc. drastically decrease, even if the molar ratio n2 of a ferrite magnet obtained by the post-addition method is controlled within the range of 5-6.
Accordingly, it is preferable to add an iron compound such as iron oxide, etc. by a post-addition method, to set the molar ratio n of a sintered ferrite magnet obtained by the post-addition method and/or the prior/post-addition method within the desired range of 5-6 without excessively increasing the molar ratio n of the calcined powder. It is preferable that the calcined powder has a molar ratio of 5-6 before the post-addition.
In addition, the post-addition method or the prior/post-addition method is advantageous for the reason that it makes easy the mass production of ferrite magnets. This is because the calcined powder of Sr and/or Ba ferrite containing no or small amounts of R elements and M elements can be used in the post-addition method or the prior/post-addition method. More conveniently, by controlling the amounts of R elements and M elements at the fine pulverization step after calcination, it is made easy to produce ferrite magnets containing R elements and M elements whose amounts differ depending on fine pulverization lots, namely ferrite magnets having various magnetic properties.